Dihydrocarbyl esters of phosphorous acid, such as dibutyl hydrogen phosphite are useful as gear lubricant additives as is or they may be further reacted with elemental sulfur to form an intermediate reactive product. The intermediate reactive product can then be reacted with an amine, olefin, or alkylene oxide to obtain a desired lube oil additive.
Processes for preparing dihydrocarbyl phosphites are well known. Alkyl phosphites may be prepared by the reaction of alcohols and alkali metal alkoxides with phosphorus trichloride and with phosphorus trioxide. However, reactions of trivalent phosphorus derivatives are known to be more complex and more difficult to control than reactions involving the corresponding pentavalent phosphorus derivatives.
Baker, et al., U.S. Pat. No. 2,670,368 and GB Patent No. 699,154 disclose the direct esterification of phosphorous acid with monohydric alcohols. According to Baker, et al., alkyl phosphites, principally dialkyl phosphites, may be produced in good yield without the formation of excessive quantities of ethers, olefins, and other undesirable products by esterification of the alcohol with phosphorous acid under such conditions that the water present and formed during the esterification is removed continuously. Continuous removal of water is achieved by the use of a solvent which forms a ternary azeotropic mixture with water and the alcohol. The phosphorous acid used in the Baker, et al. process is obtained by concentrating a more dilute acid, for example, concentrating commercial 70 percent acid whereby a solid product containing 90 percent or more H.sub.3 PO.sub.3 is usually formed.